1. Field of the Invention
The present invention relates to electrical connectors and more specifically to a device for connecting a multi-chip module to a printed wiring board.
2. Description of the Prior Art
Land grid array socket assemblies are common in the electronics industry for mounting single chip modules to printed wiring boards. The interconnection of a land grid array (LGA) module to a printed wiring board (PWB) requires the accommodation of a high area density of electronic contacts and must result in a highly reliable electronic connection over a range of operating environments. One method of interconnecting an LGA module to a PWB is by using a conductive interposer. The interposer has an array of electrical contacts on one surface which mirrors those of the LGA module and, on the opposing surface, an array of electrical contacts which mirrors those of the PWB. The mounting of the LGA module is then accomplished by aligning the electrical contacts of the LGA module, interposer and PWB and mechanically compressing the interposer.
A land grid array socket assembly using an interposer has several advantages over other more traditional methods of component mounting. The modules may be changed or easily upgraded in the field. Also, system assembly and rework costs may be reduced during production. The interposer reduces the effects of thermal expansion mismatch between the chip modules and the PWB by acting as a compliant member between the chip module substrate and the PWB surface. This compliant property of the interposer ensures electrical connectivity of the assembly over a range of thermal and dynamic operating environments.
The demand for higher performance in electronic equipment has led to the development of LGA socket and interposer assemblies for multi-chip module applications. However, the mounting of a multi-chip module presents challenges due to the greater number of electrical contacts and larger substrate size inherent with this type of electronic component.
A key challenge in using LGA sockets with interposers for multi-chip modules is the creation of a consistent mechanical clamping force to compress the interposer between the multi-chip module and the PWB. A consistent clamping force is required to ensure positive electrical connections between the components and to maintain the alignment of the assembly over various operating environments. A multi-chip module requires a high density of electrical contacts over the surface of the module substrate. This high density of contacts necessitates an initial accurate alignment of the assembly and a controlled and predictable compression force to maintain the multi-chip module, interposer and PWB in electrical contact.
Various hardware configurations have been employed to achieve the compression of the LGA socket, interposer and multi-chip module assembly. Typical existing systems use a spring member to compress the components together. The components are assembled upon the PWB and a spring member is deflected by spring actuation hardware thus clamping the components in place. One problem inherent in this approach is the range of spring deflections achieved, and hence the range of clamping forces generated, due to the mechanical tolerances presented by the assembly. The mechanical tolerances of the actuation hardware, multi-chip module, interposer and printed wiring board all directly effect the spring deflections generated in the complete assembly.
One example of an existing system for securing a multi-chip module in an LGA socket connection upon a printed wiring board is shown in FIG. 1. In such a system, a multi-chip module body 110 includes a substrate portion 112, upon which a plurality of integrated circuit chips are mounted, and a housing, which typically includes a heat sink. The substrate portion 112 is mounted upon a printed wiring board (PWB) 116 using an interposer 114. An interposer 114 is a thin sheet with a plurality of electrical contacts, arranged to mirror the electrical contacts of the substrate 112 and the printed wiring board 116, passing therethrough that facilitates electrically coupling the substrate 112 to the printed wiring board 116. The multi-chip module 110 is clamped into position by load posts 120, spring elements 122, and actuation nuts 124. The spring elements act upon a load plate 118 positioned on the underside of the PWB 116. As the actuation nuts 124 are tightened, the spring elements 122 are compressed between the load plate 118 and the actuation nuts 124. The actuation nuts 124 create a tensile load on the load posts 120 and the load plate 118 is compressed up against the PWB 116. The tensile load in the load posts results in a downward force on the multi-chip module body 110 which compresses the substrate 112, interposer 114 and PWB 116 together.
The spring actuation hardware typically includes a threaded actuation member which is used to compress the spring member. To compress the spring, the clearances in the assembly are first removed by advancing the actuation member. The actuation member is then further advanced a given number of turns to create a known deflection of the spring member. One source of uncertainty in this approach is that the determination of when the tolerances have been removed from the assembly is a subjective judgment. A second source of uncertainty is associated with monitoring the turn count of the actuation member. The end result is an imprecise displacement of the spring element and a resulting uncertainty in the compressive force applied to the multi-chip module, interposer and PWB assembly.
As further demonstrated in FIG. 1, existing systems apply the compressive force about the periphery of the assembly only. This non-uniform application of force results in an uneven deflection of the multi-chip module substrate 112, PWB 116 and interposer 114. This deflection of the components allows a corresponding variance in the compressive force seen by the individual electrical contacts across the surface of each component. The result is that the electrical contacts at the center of each mating component face are not as tightly compressed as the electrical contacts about the edges of the assembly, demonstrated by arrows 126. This variance in contact pressure reduces the integrity of the electrical connection when exposed to a range of operating environments.
Therefore, there is a need for a device that predictably applies even force to a multi-chip module and a printed wiring board.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is an apparatus for applying force to a multi-chip module having a substrate, a printed wiring board having a first side and an opposite second side and an interposer. The interposer facilitates electrical contact between the substrate and the printed wiring board through the interposer. The multi-chip module and the interposer are disposed on the first side of the printed wiring board.
The apparatus includes a plurality of elongated spaced-apart load posts, a load transfer plate, a spring member, a backside stiffener plate, and a spring actuator. The load posts are affixed to the multi-chip module and pass through a plurality of post holes defined by the printed wiring board from the first side to the second side of the board. Each load post has a proximal end affixed to the multi-chip module and an opposite distal end that defines an engagement surface. The load transfer plate is disposed opposite the multi-chip module and spaced apart from the second side of the printed wiring board. The load transfer plate defines a plurality of openings through which the distal ends of each of the plurality of load posts pass. Each of the plurality of openings is shaped so as to engage the engagement surface of the distal end of a corresponding load post.
The spring member is disposed adjacent the load transfer plate between the load transfer plate and the printed wiring board. The backside stiffener plate is disposed between the spring member and the printed wiring board. The spring actuator is engageable with the spring member so that the spring actuator applies force to the backside stiffener plate, thereby causing the substrate, the interposer and the printed wiring board to be held in contact. The spring member has a stiffness which is substantially less than the stiffness of either the load transfer plate or the backside stiffener plate.
In another aspect, the spring member includes a plurality of similarly shaped spring plates placed in vertical alignment. Each spring plate defines a bushing hole passing through the body of the plate having a size sufficient to receive a portion of the spring actuator within the opening. The bushing hole is chamfered where the hole intersects the upper and lower surface of the spring plate. The spring plates include a center portion and a plurality of spaced-apart cantilevered beams extending radially from the center portion. Each beam ends in a beam end. Moreover, the shape of the cantilevered beams is chosen such that the load verses deflection curve for the spring plate is substantially linear.
Each spring plate further includes an alignment hole passing through each beam end. The load transfer plate has an inward surface and a plurality of truncated corners with an alignment pin positioned inwardly from the inward surface at each of the truncated corners. Each alignment pin is disposed so as to be in alignment with a different alignment hole of a beam end. The spring member is positioned over the alignment pins, upon the load transfer plate. A plurality of clips are affixed to a different one of the truncated corners and are shaped to hold a different beam end adjacent to a corresponding truncated corner.
The spring element defines a bushing hole passing therethrough and disposed substantially central to the spring member. The spring actuator includes an elongated actuation screw having a first end with a tool engagement portion extending therefrom. The actuation screw has a second end with a threaded portion extending therefrom and a screw collar disposed between the tool engagement portion and the threaded portion. The spring actuator also includes a bushing with an outer surface and defines a threaded passage passing longitudinally therethrough that is complimentary to the threaded portion of the actuation screw. The bushing also has a bushing collar extending outwardly from the outer surface. The bushing is positioned within the bushing hole of the spring element and the threaded actuator positioned within the bushing.
In yet another aspect, the substrate has a substrate thickness tolerance. The printed wiring board has a first side and an opposite second side and has a printed wiring board thickness tolerance. The interposer also has an interposer thickness tolerance. The deflection of the spring member is of a magnitude of at least eight times a total stacked tolerance. The total stacked tolerance includes the sum of the substrate thickness tolerance, the interposer thickness tolerance, the printed wiring board thickness tolerance, the load post length tolerance, and the actuator length tolerance.
In yet another aspect, the apparatus also includes a backside stiffener plate, or a load transfer plate, or both a backside stiffener plate and a load transfer plate. The backside stiffener plate is disposed between the spring member and the printed wiring board and has a backside stiffener plate thickness tolerance. The load transfer plate is disposed opposite the multi-chip module and is spaced apart from the second side of the printed wiring board. The load transfer plate is coupled to the distal ends of the load posts and has a load transfer plate thickness tolerance.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.